The invention relates to a catamaran having two spaced apart substantially parallel demihulls connected by a deck structure above the waterline. A tunnel is formed between the two demihulls in which, below the waterline, transverse hydrofoils are arranged.
It is known that the resistance of planing catamarans can be improved by arranging hydrofoils in the tunnel between the demihulls such that the foils carry a certain part of the weight of the boat. Such arrangements are described in European patent applications Nos. 015 073 and 0 094 673, with the latter describing a pair of hydrofoils in a tandem arrangement with the mainfoil placed close to the center of gravity and a trimfoil disposed near the stern. The pair of foils are arranged in such a way that their running orientation or level is parallel to the water surface when the catamaran has adjusted itself to a trim angle at running speed. The combined lift forces of all foils have to act close to the longitudinal center of gravity.
Although this foil arrangement has proven successful, it still has certain disadvantages. Specifically, at the beginning of the planing condition, the catamaran will take a certain trim angle which increases the pressure resistance of the craft. Before the catamaran has been raised partially out of the water by the dynamic lift forces, an extraordinary increase of resistance (known as hump resistance) occurs. At higher speeds of the catamaran, the trim angle decreases again and the resistance increase is less steep.
The arrangement of hydrofoils according to European patent application No. 0 094 673 still has a relatively high hump resistance due to the fact that the mainfoil is located in front of the longitudinal center of gravity and carries a higher weight than the trimfoils. This causes the induced downwash behind the mainfoil to be relatively strong, especially at low speed. With increasing speed the downwash decreases in relation to the velocity of the flow past the foil. The downwash changes the angle of attack of the trimfoil at the stern. The angle of attack of the trimfoil is then decreased and the lift forces of the trimfoils are reduced, causing a further increase of hump resistance. For smaller catamarans with a high reserve of engine power this fact is not so important.
However, with larger craft having design speeds not much higher than the hump resistance speed, it is absolutely necessary to prevent any increase in the hump resistance.
It is therefore an object of the present invention to provide a catamaran having improved hump resistance characteristics achieved by a hydrofoil arrangement.